Lighting
Aquarium Lighting Calculator
Work out the lighting power your planted aquarium needs, in both watts and lumens per litre, based on whether you're growing easy plants or demanding reds and carpets.
Aquarium Lighting Calculator
Easy plants: anubias, java fern, vallis, hardy stems, mosses, crypts. No CO2 needed.
Recommended wattage
25 - 50W
- W / L range
- 0.25 - 0.5
- Lumens total
- 1,500 - 2,500 lm
- Lumens / L
- 15 - 25
How aquarium lighting is sized
Lighting a planted tank is about matching the light intensity (not just wattage) to what your plants can use. Too little light and plants melt, too much and algae outcompete them. The numbers below are the ranges UK aquascapers and planted-tank retailers actually use.
- Low tech: 0.25-0.5 W/L, 15-25 lm/L. Easy plants, no CO2, minimal maintenance.
- Medium tech: 0.5-1.0 W/L, 25-50 lm/L. Faster growth, liquid ferts, optional CO2.
- High tech: 1.0-2.0 W/L, 50-100 lm/L. Reds, carpeting plants, pressurised CO2, daily ferts.
Watts vs lumens vs PAR
Watts is the electrical power consumed - a very rough guide only, because modern LEDs produce much more light per watt than old T5/T8 bulbs. Lumens is the perceived brightness and is useful for comparing modern LEDs. PAR is what plants actually photosynthesise with, and is the gold-standard measurement - but needs a meter most hobbyists don't own. For home planting, our watts/lumens-per-litre ranges are plenty.
Deep tanks need more light
Light drops by roughly half every 30 cm of water. A 50 cm deep tank needs noticeably more intensity at the surface than a shallow 30 cm tank to deliver the same PAR at substrate level. If you're at the top of your W/L range and still struggling to grow carpets, the tank is probably too deep rather than too dim.
Frequently asked questions
How many watts of LED lighting do I need per litre?
Modern LEDs are far more efficient than the T5/T8 bulbs the old "1 watt per litre" rule was based on. As a rough guide, 0.25-0.5 W of LED per litre is low-light, 0.5-1.0 W is medium-light, and anything above 1 W per litre is high-light territory. Lumens and PAR give a much more accurate picture than watts.
What is PAR and do I need to measure it?
PAR (Photosynthetically Active Radiation) is the light intensity plants actually use. It's more accurate than watts or lumens, but you need a special meter to measure it. For most home aquariums, the watts/lumens-per-litre guide is accurate enough if you choose a reputable brand and follow their recommended mounting height.
How long should my aquarium lights be on?
Start at 6 hours a day and only extend if plants are healthy and algae is absent. 6-8 hours suits most low-tech tanks; 8-10 hours is typical for high-tech CO2-injected planted tanks. More than 10 hours almost always favours algae.
Do I need special plant-spectrum bulbs?
A full-spectrum white LED (6500 K-8000 K) covers the entire PAR curve well. "Plant spectrum" bulbs with extra red/blue push plant growth slightly and look more saturated, but aren't essential unless you're chasing reds or running an iwagumi.
Is my tank too deep for standard aquarium lighting?
Light intensity drops quickly with depth - roughly halved every 30 cm of water. Tanks deeper than 50 cm usually need higher-rated LEDs or pendants hung a little closer. If carpeting plants melt or struggle in an otherwise-healthy setup, depth is almost always the issue.
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